The invention relates to a method for detection and correction of nonlinearities in radio-frequency voltage controlled oscillators, and in particular, to a method for detection and correction of nonlinearities in radio-frequency voltage controlled oscillators such as those which are used in the form of microwave oscillators for radar applications in motor vehicles.
Radar technology is particularly suitable for use in rugged environments in motor vehicles or in industry for the purpose of contactless detection of the distance, speed, condition presence or the like of objects. The functionality, measurement accuracy and costs of radar sensors in this case depend essentially on the modulation method used, and on the associated radar signal processing.
With the FMCW (frequency modulated continuous wave) radar principle which is frequently used for distance measurement, the quality of the measured value depends on the frequency accuracy and stability of the microwave oscillator. In practice, these variables are influenced in particular by temperature drift, noise and nonlinearities in the oscillator, and therefore, generally, need to be monitored.
Contactless distance and speed measurement by radar has been known for a long time, and originates from military technology. EP 0 727 051 B1 describes a conventional FMCW method for operation of a radar works, a frequency-modulated radar signal is transmitted and is received with a phase and/or frequency shift. The measured phase and/or frequency difference, which is typically in the kHz range, is proportional to the object distance, provided the frequency modulation is carried out linearly with time. In practice, this precondition is often satisfied only inadequately.
The nonlinearities in frequency modulation are primarily caused by the voltage controlled oscillators, since, by virtue of the components used, they have a non linear voltage/frequency characteristic. Furthermore, these oscillators have phase noise which is pronounced to a greater or lesser extent and whose frequency is considerably higher than the voltage-dependent non linearity and it can generally be ignored for short distancesxe2x80x94the so-called correlation length of the radar.
On the other hand, compensation for the voltage-dependent nonlinearities is absolutely essential in order to allow correct object detection to be carried out using the radar device. Since the nonlinearities vary, for example due to temperature or aging effects, any correction needs to be continuously adapted in order to keep the linearity errors within a maximum tolerance band of 1% of the measured value.
Appropriate correction methods are known from the prior art:
The oscillator can be actuated using a control voltage which is predetermined once and is subjected to pre-emphasis. However, the suitability of this method is limited since the concept does not compensate for differences between individual oscillators or for subsequent frequency fluctuations and a temperature drift.
The document WO 98/37705 discloses a dynamic self-adjusting synchronization system for a voltage controlled oscillator, in which the output signal of the oscillator is divided to a target frequency band whose frequency is lower than that of the oscillator output signal. Frequency counter values are accumulated by a counting device during a gate time which can be selected, in such a manner that a maximum counter value is reached at the maximum frequency of the oscillator output signal. The respective counter value is converted to an equivalent-voltage value by means of a microprocessor. Finally, a control circuit produces a control signal with a corrected frequency response for the oscillator, by logic-linking the equivalent-voltage value to a predetermined nominal value.
For level measurement using radar technology, the document U.S. Pat. No. 5,799,534 discloses a method in which the output signal of a voltage controlled oscillator is likewise divided down to a target frequency band, whose frequency is lower than that of the oscillator output signal. Frequency counter values are detected by means of a counting device. These counter values are then converted to an equivalent-voltage value. A control signal with a corrected frequency response for the oscillator is produced as a function of this value.
The specialist article by P. Lowbridge et al. xe2x80x9cA low Cost mm-Wave Cruise Control System for Automotive Applicationsxe2x80x9d in Microwave Journal, October 1993 describes the use of a control loop which comprises a PLL-(=Phase Lock Loop) or AFC (=Automatic Frequency Control) circuit. In both methods, a frequency-dependent reference voltage is produced and, in combination with a linear ramp, adapts the actuation voltage of the oscillator such that the frequency modulation profile, is linear with respect to time. However, such control electronics have the disadvantage of being too expensive and inflexible.
The specialist publication by Nalezinski et al. xe2x80x9cNovel Heterodyne 24 GHz FMCW Radar with High-Precision 2.4 GHz SAW Reference Pathxe2x80x9d in the MIOP"" 97 Conference Proceedings describes the use of a reference path within the radar arrangement, which, on the basis of its defined and precisely known delay time, corresponds to the reference signal, free of any disturbance variables, from a virtual reflection point whose distance is established by means of a nominal delay time. Actual signals with similar delay times can be corrected by analysis and evaluation of this signal. However, this method is subject to the disadvantage that the additional reference path and the evaluation unit associated with it are complex, and thus costly, to operate. The latter is unacceptable, particularly in the context of such radar sensors being used as mass-produced components in motor vehicles.
In one embodiment of the invention, there is a method for detection and correction of nonlinearities in radio-frequency voltage controlled oscillators. The method includes, for example, actuating the oscillators with a linear control voltage, dividing an output signal of the oscillator by an initial division factor by a frequency divider to a target frequency band which is at least one order of magnitude lower than that oscillator output signal, analyzing the divided output signal by a counting device with registers of the counting device being loaded with preset counts, which correspond to a linear frequency response of the output signal, and with the registers being counted down successively on the basis of the analyzed, actual profile of the divided output signal and signal sampling from a received signal being carried out when the value 0 is reached in the register.
In one aspect of the invention, the signal sampling from the received signal is initiated by triggering a sample-and-hold element inhuman analogue/digital converter by the counting device.
In another aspect of the invention, the analysis of the divided signal is carried out by detecting the count of the counting device in a gate time which can be selected, with the gate time being selected in advance such that a maximum counter value is reached at a maximum frequency of the oscillator output signal and a given initial division factor.
In still another aspect of the invention, the counter value is converted to an equivalent-voltage value using a digital/analogue converter.
In yet another aspect of the invention, successive detection of the period duration of the frequency flanks of the divided signal is carried out during the analysis of the divided signal.
In another aspect of the invention, the analysis is carried out by counting the flanks of the divided signal during a given gate time.
In yet another aspect of the invention, with the aid of a microcontroller, a linear-modulated control voltage for the oscillator is output via a pulse width modulation output of a microcontroller, a frequency error from an optimum linear frequency variation is determined for the oscillator on the basis of the analysis of the divided output signal, correction values are determined for a pulse width modulation output of the microcontroller, and a control voltage with a corrected frequency response for the oscillator is output via the pulse width modulation output PWM.
In still another aspect of the invention, the divided signal is cyclically monitored for linearity errors and, if necessary, correction of the control voltage with a corrected frequency response which is output at the pulse width modulation output.
In another embodiment of the invention, there is a method for detection and correction of nonlinearities in radio-frequency voltage controlled oscillators. The method includes, for example, actuating the oscillators using a linear control voltage, dividing an output signal of the oscillator by an initial division factor by a frequency divider to a target frequency band which is at least one order of magnitude less than the oscillator output signal; and analyzing the divided output signal by a counting device with registers of the counting device being loaded with preset counts, which correspond to a linear frequency response of the output signal, and with the registers being counted down successively on the basis of the analyzed, actual profile of the divided output signal and signal sampling from a received signal being carried out when the value 0 is reached in the register.
In another aspect of the invention, the signal sampling from the received signal is initiated by triggering a sample-and-hold element in an analogue/digital converter by the counting device.